The present invention relates in general to in situ recovery by retorting organic carbonaceous values from carbonaceous deposits, as oil from oil shale, and, in particular, to a method for producing uniform retorting gas flow throughout a retort during the extraction of organic carbonaceous values.
There are vast untapped reserves of organic carbonaceous deposits which have not heretofore been exploited because it has not been economical to do so.
One type of unexploited organic carbonaceous deposit is oil shale. Vast reserves of oil bearing shale deposits exist throughout the world. One of the biggest of these deposits is in the Rocky Mountains of the United States.
The Piceance Creek Basin in Colorado is one reserve typical of many others. In the Piceance Creek Basin oil shale averaging 25 gallons per ton is found in seams varying from 50 feet vertical thickness to over 1,000 feet. However, in the areas where the oil shale is most accessible the seam thickness is invariably less than 150 feet, normally between about 100 to about 150 feet. It is in these accessible, relatively thin seam areas where commercial interest is presently being focused.
One of the most attractive methods of extracting oil from oil shale beds is by in situ retorting. In situ retorting envisions extracting the oil by heating it sufficiently to decompose kerogen, solid organic matter in the shale, into gas, oil and carbon. The oil values are collected from the in situ retort and processed further into saleable products.
The shale is fractured to produce a chimney of broken shale; i.e., a rubble pile, and is heated in place either by establishing a combustion zone in the bed or by using a retort fluid which is sufficiently hot to do the same job. In either event a retorting fluid is used. In the first case the retorting fluid, typically air, provides the oxygen necessary to support combustion in the combustion zone (thermal decomposition of residual carbon from the oil shale providing the fuel). In the second case the retorting fluid itself provides the heat energy required to retort the shale oil values. Combinations of these two types of retorting fluids and techniques have also been proposed.
The formation of a rubble-filled chimney is needed to provide passages for the retorting fluid, good heat transfer conditions to the shale, and paths for the retorted values. A broken-up bed of oil shale which is to be retorted is called a rubble pile.
An extreme method for developing a shale rubble pile is by a nuclear explosion. A nuclear explosion vaporizes some of the shale to create a void and the energy of the blast fractures the shale which will then collapse into the void. The collapsed shale occupies a larger volume than before the explosion and therefore passages are created for the retorting fluid. A nuclear created in situ retort has a length dimension running vertically which is much larger than the diameter. Typically, the length-to-diameter ratio (L/D) of a nuclear device created retort will be 2.2/1, and often greater. But nuclear created retorts must be used where there is a considerable amount of overburden or in very deep deposits to confine the effects of the nuclear blast in the ground. As such, the nuclear approach is of limited applicability even if nuclear devices were to become generally available.
A second method for developing an in situ retort and shale rubble pile envisions excavating or undercutting a large area at the base of the oil shale seam. The resulting exposed oil shale ceiling is allowed to collapse by itself.
Theoretically it is possible to mine out almost any thickness zone under the shale to create a rubble pile having almost any desired length-to-diameter ratio. But the most accessible and attractive areas are where the seams are relatively thin. In the accessible and attractive areas the undercutting technique has resulted in very low length-to-diameter ratios. One of the reasons for this is that a considerable area is necessary for free collapse of the ceiling over the excavated area. Moreover if the length is increased to obtain a high length-to-diameter ratio, it is increased only by caving low grade oil shale which is not economical to cave and retort. Consequently, retorting is horizontal when the rubble pile is developed by this method.
A third method for creating the rubble pile and an underground retort uses conventional explosives with or without natural roof failure and collapse. With natural roof collapse, the collapse is initiated by the removal of roof support pillars. After the roof fails, explosives are detonated in the remaining roof to cause further breakage and formation of the desired rubble pile. Since this method also requires free fall of a ceiling, it too requires a large cross-sectional area below the seam to produce free fall. The result, again, is a low length-to-diameter ratio. Therefore, this method also results in retorting in an essentially horizontal direction.
In the development of rubble piles it is extremely difficult to avoid voids, say, at the top of the rubble pile, where horizontally directed retorting fluids can short circuit. The result is poor retorting efficiency. Even if the broken shale "bulks full" and there is no void at the top of the retort, differences in bulk porosity at different retort heights are probable. With differences in bulk porosity in the vertical, most of the retorting fluids will pass through the zone having the greatest porosity because the resistance to flow is less. Again, poor retorting efficiencies are the result. Consequently, it is probable that very poor retorting efficiency is to be expected in horizontal in situ retorts.
Retorting in a vertical direction overcomes the difficulties in horizontal retorting because the retort gasses must pass through all vertical zones in any event and therefore will pass through any void zones.
While vertical retorting is attractive because it overcomes the problems with channeling of retorting fluids encountered in horizontal retorting, channeling is still possible in low length-to-diameter retorts. Channeling is possible because it is not always practical to provide enough retorting fluid inlets and outlets from the retort at the locations necessary to overcome the tendency of the retorting fluid to take the path of least resistance, typically the shortest path between inlet and outlet. In other words, with a limited number of retorting fluid inlets and outlets it is necessary for retorting fluids to traverse various length paths if the entire rubble pile in the retort is to be effectively contacted by the fluids. Because the techniques heretofore proposed for developing rubble piles and in situ retorts result in rubble piles which in any vertical zone have about the same bulk permeability for the retorting fluids, the problem of selective channeling exists even in vertical retorting.
Therefore, there is a need for developing rubble piles for in situ retorting of carbonaceous material, particularly in the vertical direction, which overcomes the problem of retorting fluid channeling.